Cannot add assembly features. Parts cannot be selected.
Solution
This can happen when parts have multi-bodies. Open the part which cannot be selected with the assembly feature, expand the Design Bodies, then right mouse click on the desired Design Body and select 'Activate Assembly Body'. Return to the assembly. Try to add the assembly feature again.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: n/a
Product: SOLID_EDGE
Application: ASSEMBLY
Version: V219.0
Function: FEATURES
Ref: 001-8853778